The invention relates to a method and an apparatus for transporting time related information in a packet switched network.
When data packets are transported over a packet switched network these data packets can experience random delays. Accordingly, it is necessary to measure packet delay variations accurately and factor these measurements into the calculations to derive a time alignment between devices of the packet switched network. A conventional way of distributing time information within a wider system is GPS (Global Positioning System). Because of possible GPS jamming the applications are migrating to alternative time distribution methods and in particular methods using Network Time Protocol (NTP) and IEEE 1588 PTP (Precision Time Protocol). Applications of the Precision Time Protocol currently aim to reach a 1 microsecond accuracy for time alignment, although future applications are already targeting sub microsecond accuracy. The Precision Time Protocol PTP specifies clock models such as boundary clock BC and transparent clock TC for distributing time within a network as well as Grandmaster and Ordinary (Slave) clocks for the origin and termination of the protocol. Transparent clocks are useful, in that they allow the effects of packet delay variation (PDV) to be mitigated and thus could be key in meeting sub nano second accuracy, however they have some disadvantages associated with the manipulation of time protocol packets in general. The operation of a Transparent clock in modifying the correction field of a packet which was not addressed to it, has been described by some as a layer violation since the time stamps and delay measurement fields in the conventional time distribution protocols are embedded within the payload of the data packet. Also the security of the system is undermined even if one sends keys to all intermediate nodes of the system to allow them to unlock the contents of the payload within the data packets. Further, there is a complexity and delay impact of identifying and modifying data fields within the payload which can affect the ultimate accuracy of the time alignment.
Accordingly, it is an object of the present invention to provide a method and an apparatus to overcome the above mentioned problems and which allow to transport time related information in a packet switched network accurately without causing any layer violations.